In physical experiments, an Event “E” is often described as a digital signal edge transition from either a low level to a high level or high level to low level. For optical experiments such as Optical Time Domain Reflectometry (OTDR), optical quantum interference or Time Correlated Single Photon Counting (TCSPC), it is critical to precisely measure the occurrence of a super-event “SE” that can be defined as a combination of several “E” or the result of a logical operation between several “E” that might have been arbitrarily delayed. Generally speaking, an “E” may be considered as particle detection, for example, and an “SE” the result of the detection of a precise sequence of multiple particles with one or more detectors, for example. More precisely an example in photon counting-OTDR is disclosed in IEEE Journal of Lightwave Technology, Vol. 28, No. 6, 952-964, 2010, where laser trigger signals and detection signals are considered as events (“E”). The so-called OTDR trace showing backscattered optical intensity as a function of distance in an optical fiber network is obtained by measuring the time delay between the laser trigger signal and the detection signal. The graph shows the attenuation in the optical fiber and the discrete optical reflections that occur when two connectors are connected together or when the splice between two fiber strands is not good. Looking at this OTDR trace, a discrete reflection can be called a SE and is defined by a specific delay (τ) between the acquisition of the laser trigger signal and the detection signal. A SE occurs each time a laser trigger signal is followed by a detection signal after a delay (τ).
Usually, in Quantum Optics or physical experiments several types of solutions are used in a complementary way for precisely detecting an SE and generating a signal heralding the detection of this SE. These may be classified into three families: the first family is based on an apparatus able to detect an E with a high temporal accuracy (1), the second family is made with discrete electrical components (2) and finally the third family detects the SE with a unit process that can be programmed (3). For these three solution families, synchronous solutions have to be distinguished from asynchronous solutions. In a synchronous system, operations are coordinated under the centralized control of a fixed-rate clock signal or several clocks which enables to make logical operations. An asynchronous digital system, in contrast, has no global clock: instead, it operates under distributed control, with concurrent hardware components communicating and synchronizing on channels. Families (1) and (2) are based on asynchronous apparatus, whereas family (3) is a synchronous solution.
The first family of solutions uses highly accurate time measurement device that may be based on a Time to Digital Converter. A Time to Digital Converter (TDC) or Time Digitizer is a device for recognizing events and providing a digital representation of the time at which they occurred. Usually a TDC output is the time of arrival for each incoming pulse which is represented through timestamps, namely a file containing the time occurrence of each event with a defined resolution, which may be stored in a memory, disk drive or PC. The TDC is an asynchronous apparatus, so the time precision of such a device can be much higher than the internal clock of the computer to which the TDC is connected. TDCs are used in many different applications, where the time interval between two signal pulses (start and stop pulse) of an event should be determined. One of the classical TDC architectures is based on the coarse-fine architecture where the measurement is started and stopped, when either the rising or the falling edge of a signal pulse crosses a set threshold.
These requirements are fulfilled in many physical experiments, like time-of-flight and lifetime measurements in atomic and high energy physics (as disclosed in U.S. Pat. No. 8,822,935) where TDCs are used to provide timestamps of a detected radiation event. Additionally TDCs may be used in experiments that involve laser ranging and electronic research involving the testing of integrated circuits and high-speed data transfer. This is for example illustrated in the invention described in U.S. Pat. No. 7,624,294. Several products are currently commercialized as standalone TDCs such as Stanford Research systems SR400, Becker and Hickl TCSPC modules or ID800 provided by ID Quantique. These kinds of devices enable to provide accurate timing detection of events that can be used in set-ups such as the one described in (Zhao & al., 2015) for photon counting-OTDR experiments.
An experimental set-up presented in the prior art is illustrated in FIG. 1a. In this example aiming at realizing photon counting-OTDR experiments, an FP laser diode 025 is internally modulated with a square pulse driver 020. This is done in order to obtain high dynamic range, pure optical pulses with high extinction ratio. In order to compensate the light leakage, an acousto-optic modulator (AOM) is controlled by a Pulse Pattern Generator (PPG) 015. In that case, the time of occurrence of the single photon detector's 010 output signals and the triggering signals (generated by the pulse driver 020) for the optical pulses are collected by a TDC 005 which provides timestamps of those two types of events. Those timestamps are analyzed by a data acquisition terminal connected to the TDC in order to determine the delay between laser and detection events. The OTDR trace, which is shown on the Data Acquisition Terminal 030 graphical interface, corresponds to the statistical distribution of the delay between consecutive laser and detection events. As mentioned before, a ‘SE’ in the case of the photon-counting OTDR can be e.g. a peak in the distribution of the delay that represents a discrete reflection in the fiber under test at a distance related to the value of this specific delay.
More generally, an SE may be detected through the processing of timestamps that are generated by a TDC. Then, a heralding signal of these SE can be generated by an event generator. In order to do so, the set-up used may be described with the following general principles:                A TDC is used to acquire the time of occurrence of events. These event times are stored as E timestamp values in one or more files.        SE may be detected by processing the E timestamp files in a programmable processing unit (e.g. a computer) connected to the TDC.        Every SE has a specific time occurrence that may be registered as a SE timestamp in a file.        This SE timestamp file is sent to a digital signal generator that generates a signal for each SE detection. This generator may arbitrarily delay this signal.        This enables to herald a SE detection with an accurate time definition (given by TDC resolution) and in a reconfigurable way (because the processing is made in programmable processing unit). Nevertheless, the file processing performed by the processing unit to analyze the E timestamp files and generate the SE timestamp file takes quite a lot of time. Therefore, it does not permit the latency between a SE detection and the corresponding heralding signal generation to be below 1 micro-second (˜1 μs).        
The second family of solutions is based on combinations of discrete electrical components (e.g. logical gates, electrical wires . . . ) used for detecting SE occurrence and heralding it. This is for example described in (Riedmatten & al., 2003) and (Marcikic & al., 2003) publications. These set-ups represented in FIG. 1b are realized to observe quantum interferences with photons coming from different sources. These articles disclose experimental realizations where:                the E are generated by the electrical signal heralding the emission of an optical laser pulse (‘laser clock’) 040 or the detection of single photons by Ge-based detector ‘D1’ 045,        the delay between the events generated by the laser and the ones generated by the detector 045 is adjusted with optical fibers or coaxial cables length in such a way that an event of D1 due to a photon detection reaches an AND-logical gate at the same time than the corresponding laser event. The SE is defined as both events arriving at the same time on the logical gate (which might be called a coincidence between the two events). The output signal of the AND-gate is a ‘1’ when this SE occurs. This output signal is the signal heralding the detection of the SE.        This heralding signal of the SE is sent to the InGaAs detectors D2 055 and B 060 for their activation. Appropriate delays might be added on the heralding signals in order to activate D2 and B at appropriate time when photons impinge on these detectors. This way to proceed allows one to reduce strongly the noise of D2 and B because they are activated only when the photons are supposed to reach them.        Logical gates such as OR, AND, NAND, NOR or XOR which are implemented as discrete components are used to make logical operations on several events and thus enabling SE detection. These gates are often implemented with components such as diodes, transistors or inverters        
Note that in this case, thanks to the use of logical gates made of discrete components, the precision on the detection time of the SE is quite high and the latency of the generation of the heralding signal is pretty low. This high performance is achieved because those two parameters are limited by the discrete component bandwidth and latency. However, if you want to change the SE type (e.g. we want to change the logical operation or the delay between the events before the logical operation), you need to change the discrete components or the cable length. Therefore, this method is not reconfigurable.
The third and last family of solutions that may be use for detecting SE and generating a signal heralding this detection is based on synchronous reconfigurable solutions made with synchronous processing units like e.g. micro-processors or FPGA. The exploitation of these synchronous devices allows one to perform the acquisition of digital signals E, the detection of SE and the generation of a signal heralding this detection. All these operations are performed at the given working frequency of the used synchronous device. This approach permits one to perform reconfigurable logical operations with a low latency; nevertheless it has a temporal precision that is limited to about one period of processing unit working frequency. This means that the temporal precision of the generation heralding signal which is the result of the logical operations can not be better than the period of the synchronous device.
As it has been presented here, actually there is no solution capable to generate a heralding signal after the detection of a SE with a high temporal resolution (which is less than 100 ps), a low latency (which is less than 1 μs) and in a reconfigurable way simultaneously. This main drawback might be critical in some quantum optics experiments and other research applications.
Non-Patent Literature Includes:
    De Riedmatten H., Marcikic I., Tittel W., Zbinden H., and Gisin N., (2003). Quantum interference with photon pairs created in spatially separated sources, Phys. Rev. A 67, 022301    Marcikic I., De Riedmatten H., Tittel W., Zbinden H., Gisin N. (2003). Long distance quantum teleportation of qubits from photons at 1300 nm to photons at 1550 nm wavelength. Nature 421, 509-513    Zhao Q., Xia L., Wan J., Jia, T. Gu, M., . . . Wu P. (2015). Long-haul and high-resolution optical time domain reflectometry using superconducting nanowire single-photon detectors. Scientific Reports, 5, 10441.